800€ for hobbies and 900€ for two cars also seem expensive to me. Somewhere you will have to save when the two children arrive. Maybe with the house the commuting costs will reduce massively, that would be a start.
I would primarily worry about the income with children. If a lot of commuting still needs to be done, it is hardly possible to reconcile two full-time jobs and children. Typically, one would have to work only part-time. Additionally, Cologne has daycare costs that are very high. With your income, children under two years cost over 500€ plus meals per month. Not much remains from the second part-time salary. Here you really need to be honest with yourselves about how you estimate your income for the next 10 years. In any case, the tough period will be longer than the 14 months of parental allowance.
For the renovation costs, I would have expected a list with trades and costs. Without any, I would definitely not estimate under 2000€/m². Here too, Cologne is certainly not the cheapest place.